are your brakes pre-2012? are they less than 2 years old?
if so, get them back to SRAM through your local dealer, they will cover this issues under warranty
as I mentioned in a previous post on this thread, there was an issue with the material used for the MC diaphragm on pre-2012 Avid brakes (not the pad contact adjuster as many were led to believe), it basically develops pin-holes leaks under hard use, and no matter how well you bleed the brake, it will soon leak and air will enter the MC and brake system :(
the new 2012 brakes have a new material in MC, and mine have been stellar, definitely a reliable brake
The brakes are from 2009 however I rebuilt them this spring with both lever and caliper service kits that included replacing the master cylinders. I am not 100% sure that it is the pad contact adjust but notice that I seem to develop some squish as I spin them to accomodate pad wear.